To increase the storage capacities of optical discs, the size of a light beam spot to be condensed by an objective lens provided for an optical head has been decreased year after year by increasing the NA (numerical aperture) of the objective lens and shortening the wavelength λ of the light emitted from a light source. Meanwhile, to further increase the storage capacities of optical disc media, a multilayer recording medium with multiple recording layers has also been proposed just lately.
In a conventional multilayer recording medium, a number of spacers with mutually different thicknesses are alternately stacked between its multiple recording layers, thereby minimizing a multiple reflection (see Patent Document No. 1, for example). FIG. 2 illustrates a conventional multilayer recording medium as disclosed in Patent Document No. 1.
In the multilayer recording medium shown in FIG. 2, eight information recording layers L0, L1, . . . and L7 are stacked in this order one upon the other so that the L0 layer is located most distant from the scanner (i.e., optical pickup) and that the L7 layer is located closet to the pickup, and seven spacers with thicknesses t0 through t6 are interposed between those information recording layers. Also, to make the intensity reflectances R(n+2) and R(n+3) of the respective reflective films of L(n+2) and L(n+3) layers, which are located shallower than an L(n) layer, satisfyR(n+2)×R(n+3)<0.01,the inequality t1>t0>t3=t5>t2=t4=t6 needs to be met. That is why settings can be made so that t6=t4=t2 and t5=t3, and therefore, the number of different kinds of spacers to provide to reduce the multiple reflection can be reduced from seven to four.
On the other hand, when such a conventional multilayer recording medium is scanned, the deepest layer (i.e., the layer farthest away from the optical pickup) and the shallowest layer (i.e., the layer closest to the optical pickup) will transmit the incoming laser beam at mutually different transmittances, thus making the best readout powers on the respective recording layers different from each other in some cases (see Patent Document No. 2).